Magnetic separator with a combination field



May 14, 1968 F. FRAAS 3,382,977

MAGNETIC SEPARATOR WITH A COMBINATION FIELD Filed March 8, 1965 2Sheets-Sheet 1 INVENTOR FOSTER F/MAS A'IT NEYS' F. v FRAAS May 14, 19683,382,977

MAGNETIC SEPARATOR WITH A COMBINATION FIELD Filed March 8, 1965 2Sheets-Sheet 2 w m v V M N i 3w; oz 3 9. oz 53 Sum oz PC9234 O0 oowi 55Bub oz 53 Sui oz 64m wtzmi MCJOQOII ("A-3903M NO 031031-100 uouovaa) MINVENTOR FOSTER FRAAS ATIOR EYS United States Patent 3,332,977 MAGNETICSEPARATOR WITH A COMBHNATION FIELD Foster Fraas, Hyattsville, Md.,assignor to the United States of America as represented by the Secretaryof the Interior Filed Mar. 8, 1965, Ser. No. 438,132 7 Claims. (Cl.209214) ABSTRACT OF THE DISCLQSURE Beneficiation separation of mineralsubstances having relatively low magnetic susceptibilities isefI'ectuated by passing the substances in a stream through anon-tractive, homogeneous magnetic field for a magnetizationpretreatment, and directly thereafter passing the stream of pretreatedsubstances through a tractive non-homogene ous magnetic field forseparation of the more magnetically susceptible substances from thestream.

The present invention relates to improvements in a method and meanswhich enlarge the capability of a magnetic separator to economicallyseparate minerals having almost identical magnetic susceptibilities, andto provide for the separation of minerals of lower magneticsusceptibility. These improvements are achieved by forming and arrangingthe means producing the magnetic fields such that a stream of mineralparticles introduced to the separator is first exposed to a strongmagnetization eifect in a homogeneous magnetic field of high intensitywherein the effective magnetic property of the particles is increased,and thereafter exposed to a nonhomogeneous field wherein particles witha magnetic susceptibility higher than a predetermined value, areseparated from particles with a lower magnetic susceptibility.

Heretofore magnetic separators have operated in accordance with theprinciple that particle pretreatment required the application of forcesderived from nonhomogeneous magnetic fields. In such operations feiroand paramagnetic particles have forces directed on them in the directionof higher magnetic field intensity, and diamagnetic particles haveforces directed on them in the opposite direction. It is wellestablished that nonhom-ogeneous fields produce forces which act toalter the position of the treated particles, and as disclosed in PatentNo. 2,065,460, issued to F. R. Johnson on Dec. 22, 1936, cause apro-alignment of such particles before they enter a leaving point, orfinal separation area. Also characterizing the use of a nonhomogeneousfield in particle pretreatment is a stratification of the particles intolayers due to the magnetic particles being turned around their axis oneor more times so as to be worked out of the nonmagnetic material, in amanner more fully described in Patent No. 765,013, issued to F. J. Kingon July 12, 1904. Actually, two processes occur simultaneously whenparticles are thus pretreated, namely a force or particle positioningprocess resulting from the nonhomogeneous field, and a magnetizationprocess whereby the magnetic field increases the efiective magnetism ofthe particles. The latter is a rate process, the degree of magnetizationdepend-ing on the time that particles are in the magnetic field.

The advantageous mineral separating operation secured 3,382,977 PatentedMay 14, 1968 by means of the present invention results from theseverance therein of the aforesaid force and magnetization processesinto distinct stages. Magnetization for pretreating the particles isobtained in a homogeneous field applying no perceptible tractive orposition disturbing forces on the particles, and separation is obtainedin a nonhomogeneous field. Consequently, the magnetization preliminaryto separation can be controlled to an optimum selectivity value, and theparticles pass through the pretreatment field unrestrained with noaccumulation so that each particle receive-s the same amount ofmagnetization. Because of this optimum selectivity and uniformity ofmagnetization of the particles made possible by the present invention,any separator in accordance therewith is responsive to a more preciseregulation of its operational characteristics, including currentstrength in the windings ofits field coils, and speed at which theparticles are moved through its pretreatment field, to accomplish aseparation of par-ticles having nearly the same magnetic susceptibility.

It is an object of the present invention therefore to provide for use ina magnetic separation of particles, a method and apparatus in which fineoperating adjustments are effective to cause the separation of particleswhose similar magnetic properties would otherwise make difficult theirseparation by magnetic means.

A further object of the invention is to provide for use in a magneticseparation of particles, a method and apparatus in which a combinationof homogeneous and nonhomogeneous magnetic fields is applied tosuccessively magnetize and separate the par-ticles processed thereby.

These and other objects of the invention will more fully appear from thefollowing detailed description when read in connection with theaccompanying drawing wherein:

(FIG. 1 is a partly schematic view of an exemplary form of the magneticseparator improvement according to the present invention;

FIG. 2 presents a number of graphical representations of operationalcharacteristics of the invention in its application to the processing ofa number of different minerals;

FIG. 3 is a partly schematic view of an additional embodiment of theinvention; and

- \IG. 4 is .a partial view of FIG. 3 taken along line 4--4.

Reference to FIG. 1 provides a simplified showing of basic elements in across-belt type magnetic separator 10, comprising the improvement of thepresent invention. A more detailed disclosure of a cross-belt magneticseparator can be found on page 213 of US. Bureau of Mines Bulletin No.425. Separator 10 includes a magnetic pole 11, whose projecting face orpole cap is distinguished by a Vshaped notch 12, situated between broadand narrow fiat surfaces 13 and 15 of the cap, which creates along oneend of this cap a wedge shaped conformation 16, where'on lies surface15. Pole 11 is cooperatively related with an oppositely magnetized pole13, whose projecting face or pole cap comprises an essentially flatsurface 19 arranged to lie parallel to surfaces 13 and 15, andappropriately spaced therefrom to accommodate a particle conveyingstructure tor operation between the poles. This conveying structureincludes a conveying means 22, such as a moving belt or vibratingchannel, which functions to conduct feed particles 24 between thesurfaces of the spaced pole caps in the direction indicated by the arrowin 'FIG. 1. The particles are thus first moved through a homogeneousmagnetic field between surfaces 13 and 19, and thereafter through anonhomogeneous or particle separating magnetic field generally definedin and about the area between surfaces 15 and 19. Further comprising theconveying structure is a cross-belt 26, serving to transport to aseparate bin the particles attracted to the wedge 16 as a result of theaction in the nonhomogeneous magnetic field.

All particles are processed in the homogeneous magnetic field for thesame length of time, namely the interval required for a particle to movepast the outer edge of surface 13 and arrive under the inner edgethereof at notch 12. Obviously, this time interval can be set as neededby means of a suitable speed control for conveying means 22. It is alsoevident that the effective span of surface 13 may be varied as needed topredetermine in the first instance the time during which the particleswill be subjected to magnetization in the homogeneous field. A zerofield gap existing between surfaces 13 and 15, due to presence of anotch 12, can also be varied to predetermine the degree of preliminarymagnetization of the particles prior to a separation action thereon.Without this preliminary magnetization, a field magnetization wouldoccur only in the particle separating nonhomogeneous field. Since thepath for each separating particle would then be random, the actualmagnetization would depend on the various random magnetic fieldintensities through which a particle passes. The spectrum of the numberof separated particles versus ampere turns in the magnet would thereforebe spread over a broad range. With a preliminary homogeneousmagnetization field, the magnetization is fixed and substantially thesame for every particle having the same composition. As a result, thetractive force in the nonhomogeneous field is effectively the same forevery such paramagnetic particle, and the spectrum of the number ofseparated particles versus ampere turns in the magnet is confined to anarrow range.

To facilitate a substantive demonstration of the effectiveness of apreliminary magnetization in a homogeneous field, the apparatus of FIG.1 was adapted to have its pole 11 rotatable such that it could be movedthrough an angle of 180 about a vertical axis. With this arrangementparticles 24 moving on conveying means 22, as indicated by the arrow,could be passed through the nonhomogeneous field between surfaces 15 and19, without a preliminary magnetization. In FIG. 2 are graphicallyillustrated the results of runs made to process four different mineralswith and without employing a preliminary magnetization in a homogeneousfield. The term field in the explanatory legends of the graphs signifiesthat the feed was first passed through the homogeneous field, whereasthe phrase no field signifies that the feed passed first through thenonhomogeneous field. Further, the terms Slow and Fast found on thegraphs, signify the speed at which the particles were moved through thefields and they correspond to passage velocities of 3.6 and 5.5 inchesper second.

Examination of the curves shows tha the slope AW/AH, where AW is anincrement in the amount collected at the Wedge when an increment AH isadded to the magnetic field strength, is much greater when a preliminaryhomogeneous magnetization field is used, particularly for fast passagevelocities. A large value for AW/AH represents a spectrum confined to anarrow range, and accordingly a high selectivity in particle separation.FIGURE 2 also illustrates that the use of a preliminary homogeneousmagnetization field permits separation at lower magnetic fieldintensities, thus providing for an extension of the magnetic separationrange tolower paramagnetic susceptibility values. The maximum fieldintensity in magnetic separators is limited by the permeability of thealloys used in the magnetic circuit.

The rate process wherein the degree of magnetization depends on time andintensity is different from particles of dilferent composition. Thus themagnetization difference between two particles of different compositionmay vary according to the preliminary homogeneous magnetization fieldwhich is selected. This is illustrated in Table 1 in the separation ofilmenite and hematite with two types of separation. In run 1, separationis obtained without a preliminary magnetization field, while in run 2 apreliminary magnetization field is used. Although the amount ofcollected fraction is the same, the composition of the concentratevaries according to the amount of magnetization in a preliminaryhomogeneous magnetization field, the respective values being 39 and 60percent hematite.

TABLE L-SEPARATION 0F HEMATITE FROM ILMENITE Fraction attracted toward 1Minus plus 65 mesh mixture of hematite and ilmenite. 2 Both separationsat fast passage speed of 5.5 inches per second.

Although this invention has been described with the use of a cross-beltseparator as an example, it is applicable to a variety of otherstructural forms wherein a combination of homogeneous and nonhomogeneousmagnetic fields may be utilized. The specific characteristics of adesign for any such form would depend upon whether the particles arecarried through the fields by fluids, belts, vibrating conveyors, rolls,or by freefall.

Reference to FIGS. 3 and 4, provides a showing of a freefall type ofseparator comprising the present invention. Particles 29 are moved on avibrating surface 30, in the direction indicated by the arrow, to passbetween a lower magnetic pole 32 and upper magnetic pole 34. Pole 32 isfashioned to have a pole cap with an upwardly protruding frontal part 36defined by a flat, horizontal upper surface 38, which terminates at oneend as a Wedge shaped conformation 40. The protrusion 36 extends partway into a relatively deep notch in the face of pole 34, to define apassage between the poles wherein vibrating surface 30 can be madeactive to propel particles 29 through two distinct magnetic fields. Twoflat surfaces 42 and 44, constituting horizontal and vertical faces,respectively, of the notch in pole 34, meet at a right angle wherebysurface 42 is disposed parallel to surface 38 on pole 32, and surface 44is located opposite wedge 40 of pole 32. A knife-edge particle divertingdevice 46 is located below wedge 40 and surface 44, and arranged in anarea approximately between the wedge and face. In addition, a narrowcross belt 48, supported to move around a pair of spaced apart pulleysand 52, is arranged to traverse the area contiguous to the extendedsurfaces of wedge 40, for a purpose to be hereinafter more fullyexplained.

The magnetic field between surfaces 38 and 42 is homogeneous, while themagnetic field between wedge 40 and surface 44 is nonhomogeneous.Particles 29, moving along conveyor 30, therefore first receive apreliminary magnetization by passing through the homogeneous field andthereafter are affected by tractive forces in falling from the end ofconveyor 30 into the nonhomogeneous magnetic filed. Diverting device 46in the path of the falling particles is accordingly effective to divideor group these particles into distinct fractions 54 and 55, constitutedrespectively by the magnetic particles of the feed attracted towardwedge 40, and the remaining nonmagnetic, diamagnetic, or lower magneticsusceptibility particles. To safeguard against the possibility that someparticles may be attracted so strongly as to adhere to wedge 4%, crossbelt 48 is made operative 'by a drive from pulley 50 or 52, to pass infront of wedge 40 and to divert and remove any such wayward particles.

TABLE 2.CONCFINTRATION OF BERYL l\-IOUNTAIN ORE AFTER ONE PASS THROUGHSEPARATOR Weight Composition, Recovery Fraction class percent percentberyl of beryl,

percent 1 Nonmagnetic gangue minerals are feldspar and quartz.

While a preferred procedure and embodiment of the invention has beendescribed and illustrated, it is understood that the invention is notlimited thereby but is susceptible to change in form and detail.

What is claimed is:

1. A method for separating from a mass of particles of which individualparticles have magnetic susceptibilities varying from negligi le valuesto perceptible values, the particles of greater magneticsusceptibilities, comprising the steps of feeding the particles in apositively controlled stream through a homogeneous magnetic fieldwhereby the particles of the mass are subjected to a substantiallynon-tractive, magnetization treatment in said homogeneous magnetic fieldfor a finite interval of time, directly thereafter passing the treatedparticles through a tractive nonhomogeneous magnetic field wherein theparticles of greater magnetic susceptibility are removed from thestream, and collecting the removed particles.

2. In a magnetic separator adapted to separate a particle streamcomposed of a mixture of slightly magnetic particles and relativelynon-magnetic particles, a combination comprising a pair of magneticpoles arranged to face each other across an air space comprisingsequential air gaps, the first of said poles having a cap whoseconformation includes a large flat area and an area inclined to saidlarge fiat area, the second of said poles having a cap whoseconformation includes a large flat area in parallel relationship withsaid flat area of said first pole cap to obtain in one of said air gapsa homogeneous magnetic field therebetween, and a second portioncooperatively related with said inclined area of said first pole cap toobtain in a second of said air gaps a nonhomogeneous magnetic fieldtherebetween, a conveying structure disposed between said pole caps andoperative in said air gaps thereof to carry said particles of saidstream first through said homogeneous field and thereafter directlythrough said nonhomogeneous field, and means operatively related to saidconveying structure in said nonhomogeneous field to collect saidslightly magnetic particles removed from said particle stream towardsaid inclined area of said first pole cap.

3. In a magnetic separator for concentrating into fractions particulatematter of varying magnetic susceptibilities, a combination comprisingfirst means for producing a homogeneous magnetic field of prescribedlength and intensity, and 'second means for producing a nonhomogeneousmagnetic field adjacent to said homogeneous field, wherein said firstand second magnetic field producing means of said combination comprisestwo spaced apart magnetic poles, each having a flat surface disposedfacing one another in a parallel relationship, between which is producedsaid homogeneous magnetic fields, and one of said magnetic poles havingin addition a wedge shaped conformation on which a relatively narrowflat surface is disposed to face in a parallel relationship a furtherfiat surface on said other magnetic pole, whereby said narrow andfurther flat surfaces are cooperatively related to produce saidnonhomogeneous magnetic field, means conveying said particulate matteras a flowing stream in successive order through said homogeneousmagnetic field wherein said particulate matter is moved unrestrained andwithout accumulation so that each particle of said matter is equallytreated with magnetization, and directly thereafter through saidnonhomogeneous magnetic field wherein the particles of said matter ofhigher than a predetermined magnetic susceptibility are diverted into apath difierent from the path taken by the particles of said matter oflower magnetic susceptibility.

4. The magnetic separator of claim 3, wherein said first and secondmagnetic field producing means of said combination comprises two spacedapart magnetic poles, each having a fiat surface disposed facing oneanother in a parallel relationship and between which is produced saidhomogeneous magnetic field, and one of said magnetic poles having inaddition a wedge shaped conformation disposed to face a further fiatsurface arranged perpendicular to said fiat surface on said othermagnetic pole, whereby said wedge conformation and further fiat surfaceare cooperatively related to produce said nonhomogeneous magnetic field.

5. The magnetic separator combination of claim 3 wherein said meansconveying said particulate matter comprises a belt.

6. The magnetic separator combination of claim 3 wherein said meansconveying said particulate matter comprises a vibrating conveyor.

7. The magnetic separator of claim 3 wherein said means conveying saidparticulate matter to pass through the homogeneous magnetic field is avibrating conveyor so situated and controlled that said particulatematter leaves said vibrating conveyor to pass through saidnouhomogeneous magnetic field by freefall.

References Cited UNITED STATES PATENTS 1,529,970 3/ 1925 Ullrich 209-219X 1,536,541 5/1925 Ullrich 2092l9 2,056,426 10/ 1936 Frantz 2092322,338,501 1/ 1944 Followill 209--223 2,591,121 4/1952 Blind 209-2232,976,995 3 1961 Forrer 209-223 FOREIGN PATENTS 451,585 8/1936 GreatBritain.

HARRY B. THORNTON, Primary Examiner.

R. HALPER, Assistant Examiner.

